1. Field of the Invention
This invention relates generally to the field of mass storage disk controllers.
2. Description of the Prior Art
A data processing system typically includes, in addition to a central processing unit, a main memory subsystem and a mass storage subsystem. The mass storage subsystem includes a number of disk drives and a disk controller.
In the slower speed prior art systems typically having a data bit rate of 5 megahertz or less, the disk controller would assemble the data bits received from the disk device into data bytes. Blocks of data bytes would be transferred to main memory under control of the CPU.
As the disk technology improved, data bit rates in the order of 10 megahertz were feasible. To process data bytes at this rate, typical disk controllers would store bytes in a first in-first out memory. Data bytes would be transferred to main memory as the data bits were being received from the disk drive and organized into bytes.
For the higher data bit rates of the order of 15 megahertz, the disk controller included a random access memory to store a block of data bytes. The data bytes were transferred to main memory after the entire block was stored in RAM.
For the block transfer between the data RAM and main memory, prior art disk controllers include a first address register for the data RAM and a second address register for main memory. Under firmware control, each address register is loaded with their respective address. Information is transferred between the address location specified by the contents of the first address register and the address location specified by the contents of the second address register.
This technique balanced the hardware cost of the two registers against the read only memory (ROM) cost of the firmware to manipulate the contents of the two registers. Since the cost of ROM's was high, designers tried to use more hardware to reduce the firmware stored in the ROM for the address generation. Accordingly, it was economical to provide two address registers.
2. However, the cost of ROM's has decreased greatly. Today's designs lean toward the greater use of firmware compared to the use of hardware in the design.